Add example for ESP-IDF + SPIFFS usage

[maxgerhardt]

Per https://community.platformio.org/t/unable-to-build-and-upload-spiffs-filesystem-image-with-framework-esp-idf/17820.

Users are confused since out-of-the-box for a newly generated ESP-IDF project, the "Build Filesystem Image" task fails without any meaningful error messages.

Only using the Espressif docs you can figure out that you need to set a custom partition table in ESP-IDF (and PlatformIO) and use spiffs_create_partition_image(spiffs data) in the CMakeLists.txt of the project root, then "Build Filesystem Image" and "Upload Filesystem Image" will work and do the right thing, that is turn the files in the data/ folder of the project into a SPIFFS image and upload it.

The code seems to be slightly confusing here, too. E.g.,

platform-espressif32/builder/main.py

Lines 164 to 165 in a57403c

	MKSPIFFSTOOL="mkspiffs_${PIOPLATFORM}_" + ("espidf" if "espidf" in env.subst(
	"$PIOFRAMEWORK") else "${PIOFRAMEWORK}"),

explicitly recognizes ESP-IDF and changes the MKSPIFFSTOOL accordingly. But in the end, when the buildfs target is executed, it is going into the CMake build system and ESP-IDF does it's own thing I guess? Or does it read what we put in the MKSPIFFSTOOL env variable?

In any case, an example project espidf-spiffs or something would guide users to the right path when they want to do the same thing as they do in Arduino-ESP32 without any configuration.